13.2. Data Category Header

                      -(Indicates Structure Number)
                     |
          5      11  |
     ---- -- ---- ----
    |XXXX|  |XXXX|DRM1|
     ---- -- ---- ----
          |        |
          |        |_Compulsory Data Category Keyword(A4)
          |
          |_Optional User Identifier(A2)